Carrier Aggregation
The Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8 (Rel-8) standard for wireless communication systems has recently been finalized, supporting bandwidths up to 20 megahertz (MHz). LTE and High-Speed Packet Access (HSPA) are sometimes called “third generation” (3G) communication systems and are currently being standardized by the 3GPP. The LTE specifications can be seen as an evolution of the current wideband code division multiple access (WCDMA) specifications.
An LTE system uses orthogonal frequency division multiplex (OFDM) as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UEs). An LTE system has channel bandwidths ranging from about 1.4 MHz to 20 MHz, and supports throughputs of more than 100 megabits per second (Mb/s) on the largest-bandwidth channels. One type of physical channel defined for the LTE downlink is the physical downlink shared channel (PDSCH), which conveys information from higher layers in the LTE protocol stack and to which one or more specific transport channels are mapped. Control information is conveyed by a physical uplink control channel (PUCCH) and by a physical downlink control channel (PDCCH). LTE channels are described in 3GPP Technical Specification (TS) 36.211 V8.4.0, Physical Channels and Modulation (Release 8) (September 2008), among other specifications.
An IMT-Advanced communication system uses an internet protocol (IP) multimedia subsystem (IMS) of an LTE, HSPA, or other communication system for IMS multimedia telephony (IMT). In the IMT advanced system (which may be called a “fourth generation” (4G) mobile communication system), bandwidths of 100 MHz and larger are being considered. The 3GPP promulgates the LTE, HSPA, WCDMA, and IMT specifications, and specifications that standardize other kinds of cellular wireless communication systems.
In order to meet the upcoming IMT-Advanced requirements, 3GPP has initiated work on LTE-Advanced. One of the parts of LTE-Advanced is to support bandwidths larger than 20 MHz. This will be achieved using a concept called “Carrier Aggregation”, where multiple carrier components, each of which may be up to 20 MHz wide, are aggregated together. Carrier aggregation is planned for Release 10 (Rel-10) of the 3GPP LTE specifications.
Carrier aggregation implies that an LTE Rel-10 terminal can receive multiple component carriers, where the component carriers have, or at least the possibility to have, the same structure as a Rel-8 carrier. Carrier aggregation is illustrated in FIG. 1, in which 5 bands of 20 MHz each are aggregated together.
Carriers can be aggregated contiguously, like in FIG. 1, or they may be aggregated from discontinuous portions in the frequency domain, such that, e.g., parts of the aggregated carriers may be contiguous, and other aggregated carriers appear somewhere else in the spectrum, as schematically illustrated in FIG. 2.
The artisan will understand that the blocks shown in FIGS. 1 and 2 are compliant with the LTE specifications. With the carrier aggregation concept, it may be possible to support, among other things:                higher bit-rates;        farming of non-contiguous spectrum—e.g., provide high bit-rates and better capacity also in cases when an operator lacks contiguous spectrum;        fast and efficient load balancing between carriers.        
It should be noted that carrier aggregation is a user-equipment-centric concept, in that one user equipment (UE) can be configured to use, e.g., the two left-most carriers in FIG. 2, another UE can be configured to use only a single carrier, and a third UE can be configured to use all of the carriers depicted in FIG. 2.
Thus, an eNodeB (eNB) (i.e., an LTE radio base station) may be in control of all four carriers depicted in FIG. 2, but Rel-10 UEs may have different Configured Component Carriers (Configured CCs) that each Rel-10 UE is configured to use. The aggregated carriers may also be available for Rel-8 UEs, meaning that each of the carriers may be independently available for single-cell operation.
A particular and relevant example of a plausible carrier aggregation scenario includes the case when two or more Rel-8 compatible downlink carriers are aggregated for a UE. It should be noted that carrier aggregation is typically and mainly relevant for a Connected UE, which is a UE that is actively involved in transmission to and from an eNB (which can generally be a E-UTRAN base station), and thus has a connection with the eNB controlling the aggregated carriers.
Mobility and Measurements
In Connected mode, mobility (i.e., handovers between cells) is controlled by the network based on, among other things, measurements provided to the network by the UE. Based on measurement reports received from the UE, the eNB may deduce if a handover is needed. If so, the eNB may then issue a handover to another cell, possibly so that the other cell is controlled by another eNB.
Measurement configurations are controlled by the eNB, i.e., the eNB tells the UE, e.g., when to perform measurements, what to measure, and how to report. Such controlling information sent from the eNB to the UE includes, e.g., information of how measurements should be filtered, different thresholds for the triggers that trigger report, what to measure, how to report, and what to include in the report.
The Rel-8 LTE specifications support a versatile measurement model where different events with thresholds can be configured, such that the UE sends measurement reports to the network when, e.g., the relative signal strength between the current “Serving Cell” and a “Neighbor Cell” is changing, such that a handover may be necessary. This can occur, e.g., when the UE moves from one cell to another, as depicted in FIG. 3, which is a plot of received signal level vs. time or distance.
In Rel-8, the “Serving Cell” denotes the cell that the UE is connected to, while the “Neighbor Cell” may be another cell in close proximity on the same frequency (intra-frequency measurements), or on a different frequency (inter-frequency measurements). The neighbor may also use a different Radio Access Technology (inter-RAT measurements).
Rel-8 includes different event-triggers for issuing reports from the UE to the eNB, when certain conditions are fulfilled. For example, Rel-8 includes the trigger Event A3 defined as follows:                Event A3: Neighbor cell becomes amount of offset better than serving cell.        
Additional triggers and definitions can be found in the LTE specification, 3GPP Technical Specification (TS) 36.331 V8.8.0, Evolved Universal Terrestrial Radio Access (E-UTRA) Radio Resource Control (RRC), Protocol Specification (Release 8) (December 2009).
In 3GPP TS 25.331 V9.0.0, Radio Resource Control (Release 9) (September 2009), UTRAN trigger events are defined.
For clarity, we here list some of the definitions used in Clause 5.5 of 3GPP TS 36.331:
1. Measurement objects: The objects on which the UE shall perform the measurements.
2. Reporting configurations: A list of reporting configurations including e.g. the aforementioned trigger configurations.
3. Measurement identities: A list of measurement identities where each measurement identity links one measurement object with one reporting configuration.
Additional definitions can be found in Clause 5.5 of 3GPP TS 36.331, for example.
Considering Event A3, it is thus possible to configure an A3-event on a measurement object, such that if any Neighbor on that object grows stronger than the Serving cell (plus some configurable thresholds), then the UE shall send a measurement report that includes information about the measured radio environment of the UE. The report is constructed with relevant information, such that the eNB can decide if a handover is required or at least beneficial.
The Rel-8 measurement object may be the carrier “defined” by the Serving Cell, in which case the Neighbor and Serving are on the same frequency. The term “intra-frequency” is often used. Alternatively, the object may be a different, “inter-frequency” object, as illustrated by FIG. 4.
The same, or different, reporting configurations for A3 (or other) events could be configured for the two objects in the figure.
A characteristic of this Rel-8 model of relevance for the present invention is the fact that the UE has a single Serving Cell.
The s-Measure Parameter
The procedures for measuring on neighbor cells consume UE power. In Rel-8, there has therefore been defined a parameter called s-Measure, by which the measurement activities of a UE can be reduced at times when there is no need to perform neighbor cell search and measurements.
From Clause 6.3.5 of 3GPP TS 36.331:
s-Measure
Serving cell quality threshold controlling whether or not the UE is required to perform measurements of intra-frequency, inter-frequency and inter-RAT neighbouring cells. Value “0” indicates to disable s-Measure.
Thus, a UE needs to perform neighbor-cell measurements only if the quality of the serving cell is below a certain threshold. In Rel-8, the quality is evaluated in terms of the received RSRP (Reference Signal Received Power). If the received signal power from the serving cell is high, then the UE does not need to perform any neighbor cell measurements, as the current serving cell is assumed to be strong enough in absolute terms. FIG. 5 illustrates the use of s-Measure, where the curve is a schematic illustration of received signal level as a function of time or distance.
In IDLE mode, the mobility is UE-controlled, such that the UE selects the best cell to camp upon based on specified criteria and related parameters, where the parameters are typically broadcast in the cell. For LTE, this is described in, for example, Clause 5.2 of 3GPP TS 36.304 V8.8.0, User Equipment (UE) Procedures in Idle Mode (Release 8) (December 2009). This cell selection, where the UE selects one unique cell to camp on, is based on cell search and measurements. Also here, the network may broadcast parameters S-intrasearch and S-nonintrasearch, such that a UE may omit any cell search and measurements for cell selection on intra- and inter-frequency carriers, respectively, if the received quality of the present serving cell is greater than the aforementioned threshold parameters.
A problem with the measurement configuration and event triggers arises when Carrier Aggregation is introduced. Now, a UE may be “served” on multiple frequencies, and there arises an ambiguity of what the “Serving Cell” in FIG. 4 actually is. Specifically, the 3GPP RAN2 working group has recently agreed that each component carrier is a separate measurement object, as illustrated in FIG. 6.
Further reference can be made to 3GPP R2-100826: Report of 3GPP TSG RAN WG2 meeting #68 held Nov. 9-13, 2009.
In terms of the Rel-8 model, the UE now has three serving cells in the illustrated example.
Assume now that a UE is configured with three Component Carriers (CCs). With Rel-8 nomenclature, the UE in FIG. 6 would now have three “Serving Cells”. The term “Component Carrier”, or CC, may for example be defined as a downlink (DL) frequency that a UE is currently configured with, such that the UE is prepared to receive that DL carrier. In the following the terms “Component Carrier” and “serving cell” will be used more or less interchangeably.
FIG. 7 is a schematic diagram illustrating of an example of a situation when a UE is served by multiple serving cells or carrier components. FIG. 7 shows a first base station 1 and a second base station 2. The first base station 1 is currently a serving base station serving a user equipment, UE, 3 and the second base station 2 is a neighbor base station. As mentioned above, the UE 3 may be configured with multiple serving cells, or so-called component carriers, CCs, which relate to carriers on different frequencies (t).
A particular problem is that it is unclear how the s-Measure evaluation and similar evaluation of the need for neighbor cell measurements for a UE with multiple, aggregated Component Carriers should be performed.